The present application relates to techniques for controlling a tape transport using feedback of velocity and tension using controllers that depend on the longitudinal tape position.
Demand for cost effective storage solutions is being driven by the explosive growth in the rate at which data is being created. Tape systems are well suited to address this demand due to their low total cost of ownership compared to other storage technologies. However, the continued success of tape technology depends on maintaining its cost advantage over other storage technologies, and hence it is necessary to continue scaling the cartridge capacity and hence the cost per GB of tape systems at least as quickly as competing technologies, such as hard disk drive (HDD) storage systems.
Historically, the capacity scaling of tape systems at a roughly 40% compound annual growth rate (CAGR) has been enabled through continuous incremental improvements in track and linear densities, format efficiency, and increases in tape length. Although the majority of capacity improvements have been achieved through areal density scaling, tape length increases that are enabled by thinner tape media have also been an important contributor. For example, the capacity of the linear tape open (LTO) tape format was increased from 200 GB in generation 2 to 6 TB in the latest generation 7 format. Approximately 1.58 times of this 30 times capacity increase was enabled by an increase in tape length from 609 m in generation 2 to 960 m in generation 7. This increase in tape length required a reduction in tape thickness from 8.9 μm in generation 2 to 5.6 μm in generation 7 to maintain a constant cartridge form factor. The International Storage Industry Consortium (INSIC) 2012 Tape Technology Roadmap projects that in the future this trend will continue with tape thickness projected to reach 4.0 μm by the 2022 time frame (INSIC (2012)).
The tape transport, also known as reel-to-reel, control system of a tape drive has the task of determining the input motor currents that are applied to control the tape motion as the tape is streamed over the head during write and read operations. To achieve reliable recording performance, it is important to keep as small as possible the fluctuations of the tape velocity and of the tape tension around constant predetermined values. Previous work on reel-to-reel control has focused mainly on improving velocity control, as in Pantazi et al. (2014), where the control system for each reel consists of a feedback controller for tape-velocity control and a feedforward controller for tape-tension control. The trend of decreasing tape thickness has created a need to improve the performance of the reel-to-reel control system and in particular, to improve the control of the tension to avoid breaking the tape. In addition, variations in tape tension lead to variations in the tape's lateral dimension, so-called tape dimensional stability, that limit the track density that can be recorded on the tape. Hence, improvements in tension control may translate into improvements in tape dimensional stability that in turn may enable increases in track density.
Accordingly, a need arises for techniques by which a tape transport control system may provide enhanced regulation of tape tension and velocity over the entire length of the tape, thus reducing mechanical stress and enabling reduction in tape thickness.